HCMV is an ubiquitous virus that is present in over 60% of the population depending on socioeconomic status. Following primary infection, HCMV persists for the life span of the host. Although HCMV is generally benign in healthy individuals, the virus can cause devastating disease in immunocompromised populations resulting in high morbidity and mortality (for review, see (Pass, R. F. 2001. Cytomegalovirus, p. 2675-2705. In P. M. H. David M. Knipe, Diane E. Griffin, Robert A. Lamb, Malcolm A. Martin, Bernard Roizman and Stephen E. Straus (ed.), Fields Virology, 4th ed. Lippincott Williams & Wilkins, Philadelphia, incorporated by reference herein).
CMV is one of the most immunogenic viruses known. High antibody titers are directed against numerous viral proteins during primary infection of healthy individuals (Alberola, J et al., J Clin Virol 16, 113-122 (2000); Rasmussen L et al., J Infect Dis 164, 835-842 (1991); and (Farrell H E and Shellam G R, J Gen Virol 70 2573-2586 (1989), all of which are incorporated by reference herein. In addition, a large proportion of the host T cell repertoire is also directed against CMV antigens, with 5-10 fold higher median CD4+ T cell response frequencies to HCMV than to acute viruses (measles, mumps, influenza, adenovirus) or even other persistent viruses such as herpes simplex and varicella-zoster viruses (Sylwester A W et al., J Exp Med 202, 673-685 (2005). A high frequency of CD8+ responses to defined HCMV epitopes or proteins is also commonly observed (Gillespie G M et al., J Virol 74, 8140-8150 (2000), Kern F et al., J Infect Dis 185, 1709-1716 (2002), Kern F et al., Eur J Immunol 29, 2908-2915 (1999), Kern F et al., J Virol 73, 8179-8184 (1999) and Sylwester A W et al., J Exp Med 202, 673-685 (2005). In a large-scale human study quantifying CD4+ and CD8+ T cell responses to the entire HCMV genome, the mean frequencies of CMV-specific CD4+ and CD8+ T cells exceeded 10% of the memory population for both subsets and in some individuals, CMV-specific T cells to account for >25% of the memory T cell repertoire.
Paradoxically, the robust immune response to CMV is unable to either eradicate the virus from healthy infected individuals or confer protection against re-infection. This ability of CMV to escape eradication by the immune system, and to re-infect the sero-positive host has long been believed to be linked to the multiple viral immunomodulators encoded by the virus (for review, see Mocarski E S et al., Trends Microbiol 10, 332-339 (2002) incorporated by reference herein.) The HCMV US6 family of proteins (equivalent to RhCMV homologues: Rh182-Rh189) are the most extensively studied of these immunomodulators (Loenen W A et al., Semin Immunol 13, 41-9 (2001); incorporated by reference herein.) At least four different genes, US2, US3, US6 and US11—and the respective RhCMV homologues (Rh182, Rh184, Rh185, and Rh189)—are known to interfere with assembly and transport of MHC I molecules (Ahn K et al., Proc Natl Acad Sci USA 93, 10990-10995 (1996), Ahn K et al., Immunity 6, 613-621 (1997.) Jones T R et al., J Virol 69, 4830-4841 (1995); Pande N T et al., J Virol 79, 5786-5798, (2005). Wiertz E J et al., Cell 84, 769-779 (1996); and Wiertz E J et al., Nature 384, 432-438 (1996); all of which are incorporated by reference herein.)
Each of these four molecules interferes at different essential points of MHC I protein maturation. US2 binds to newly synthesized MHC I heavy chain (HC) and reverse translocates the protein through the translocation channel SEC61 back into the cytosol where HC is degraded by the proteasome. Similarly, US11 ejects MHC I back out into the cytoplasm. US3 and US6 act later in the MHC-I assembly process with US3 retaining fully formed heterotrimers in the ER thus preventing their transport to the cell surface and US6 preventing peptide transport by TAP and thus formation of the trimeric complex of HC, β2 m and peptide.
CMV-based vectors expressing heterologous antigens do not induce cytotoxic T cells directed against immunodominant epitopes of those heterologous antigens. This limits the efficacy of the T cells raised by a CMV-based vaccine to protect against infection by a pathogen or mount a cellular immune response against a tumor.
However, CMV-based vectors lacking viral inhibitors of antigen presentation by MHC class I molecules—CMV based vectors that have deleterious mutations in (including deletion of) all of US2, US3, US6, US8, US10, and US11 (ΔUS2-11 vectors) do indeed induce T cells to respond to immunodominant antigens. (Hansen S G et al., Science 328, 102-106 (2010). However, wild type US2, US3, US6, US8, US10, and US11 confer superinfectivity in wild-type CMV vectors. Therefore vectors that have deleterious mutations in all of US2, US3, US6, US8, US10, and US11 are eliminated by cytotoxic CD8+ T cells in individuals previously inoculated with CMV-vectors or naturally infected with CMV. Because the vast majority of humans have been exposed to CMV at some point in their lives, CMV based vectors that have deleterious mutations in all of US2, US3, US6, US8, US10, and US11 would be of limited use.
The ability of wild type CMV to super-infect CMV-immune individuals and its inability to induce cytotoxic CD8+ T cells to immunodominant epitopes of heterologous antigens was thought to be intricately linked. Immunogenicity of CMV vectors was only be improved at the cost of losing the ability to super-infect.
There is a need for CMV vectors that are able to super-infect CMV-immune individuals and induce an immune response, for example, cytotoxic CD8+ T cells.
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.